Optical fiber laser

ABSTRACT

An optical fiber laser including: a master oscillator; and a power amplifier, the power amplifier including: a plurality of excitation light sources; excitation ports each of which is connected to the excitation light sources and which an excitation light emitted from each of the excitation light source enters; a signal port which a laser beam emitted from the master oscillator enters; an optical coupler with an exit port that outputs the excitation lights from the excitation ports together with the laser beam from the signal port; and an optical fiber connected to the exit port, in which the optical fiber is a photonic bandgap fiber, and the optical fiber has a loss wavelength characteristic in that a photonic bandgap region is narrower than a gain wavelength band in a graph with an axis of abscissa representing a wavelength and an axis of ordinate representing a loss amount.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application based on a PCT PatentApplication No. PCT/JP2009/052918, filed Feb. 19, 2009, whose priorityis claimed on Japanese Patent Application No. 2008-038005 filed Feb. 19,2008, the entire content of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber laser, moreparticularly to an optical fiber laser in which a photonic bandgap fiberadjusted to produce a photonic bandgap region only in a signalwavelength region is used, and a parasitic oscillation is suppressed.

2. Description of the Related Art

Recent years have seen advances in the high output of fiber lasers.Optical fiber lasers with an output over kW have been developed. Suchoptical fiber lasers with a high output have come to be utilized in avariety of fields such as in finishing machines, medical equipment, andmeasuring equipment. Compared with other types of lasers, the opticalfiber lasers have an excellent capability to collect light, and hence,are capable of obtaining a very small beam spot with a high powerdensity. Therefore, the optical fiber lasers allow high-precisionmachining Machining that uses the optical fiber laser is non-contactmachining, and is also capable of machining a hard substance if thesubstance is capable of absorbing a laser beam. For these and otherreasons, the range of applications of the optical fiber lasers israpidly increasing especially in the field of material machining

FIG. 14 shows a schematic diagram of a representative high-outputoptical fiber laser with a system called the MOPA.

In the MOPA system, to the subsequent stage of a master oscillator(hereinafter, sometimes referred to as MO) 100, a power amplifier(hereinafter, sometimes referred to as PA) 200 is connected. With thisconfiguration, a feeble pulsed beam that has been output from the MO 100is amplified by the PA 200, and a laser beam with a high output isemitted from the PA 200. If a sufficient output is not obtained with asingle-stage PA 200, PAs 200 are connected in multiple stages so as toobtain a desired output.

Systems for the MO 100 include: a system in which an output of aCW-oscillating laser light source such as a semiconductor laser ismodulated in intensity with a modulator such as an acoustoopticalelement into pulsed light; and a system in which a fiber ring laser isused such as described, for example, in Patent Document 1.

FIG. 15 shows a schematic block diagram of a representative fiber ringlaser.

A fiber ring laser 100 comprises: an excitation light source 101; a WDMcoupler 102 for combining an excitation light with a laser beam; arare-earth-doped optical fiber 103, the rare earth being a gain medium;an isolator 104; an optical switch element 107; and an output coupler105. The excitation light emitted from the excitation light source 101enters the rare-earth-doped optical fiber 103 via the WDM coupler 102.The excitation light having entered the rare-earth-doped optical fiber103 is absorbed into the rare-earth ions doped in the core of therare-earth-doped optical fiber, to thereby excite the rare-earth ions.The rare-earth ions in the excited state emit spontaneous emission witha specified wavelength. While being amplified, the spontaneous emissionpropagates through the rare-earth-doped optical fiber 103, and is outputas an Amplified Spontaneous Emission (ASE). The WDM coupler 102, therare-earth-doped optical fiber 103, the isolator 104, the output coupler105, and the optical switch element 107 are connected in a ring.Therefore, the ASE circulates through these parts, and is againamplified by the rare-earth-doped optical fiber 103. After sufficientlyamplified, the ASE laser-oscillates, a part of which is output as alaser beam via the output coupler 105. At this time, if the opticalswitch element 107 is operated so as to periodically repeat a low lossstate and a high loss state, the ASE pulse-oscillates. Thus, a pulselaser output is obtained.

For the PA 200, an amplifier with a configuration as shown in FIG. 16 isused. FIG. 16 shows a configuration of an optical fiber laser with theMOPA system. The laser beam that has been output from the MO 100 entersthe PA 200 via an interstage isolator 316, and is output afteramplification by the PA 200.

The PA 200 includes: a plurality of excitation light sources 201; anoptical coupler 203; a rare-earth-doped optical fiber (rare-earth-dopeddouble-clad fiber) 210; and an isolator 206. As for the excitation lightsources 201, the rare-earth-doped optical fiber 210, and the isolator206, the same ones as those used in the MO 100 may be used. For theoptical coupler 203, an optical coupler such as described in PatentDocument 2 is used. The optical coupler 203 has: a plurality ofexcitation ports 202 made of a multi-mode optical fiber; and a signalport 204 made of a single-mode fiber; and further has an exit port 205that is formed by fusing and drawing the multi-mode optical fiber andthe single-mode fiber into an integrated entity. The laser beam emittedfrom the MO 100 enters at the signal port 204 and is emitted to the coreof the rare-earth-doped double-clad fiber 210 via the optical coupler203. On the other hand, to the optical coupler 203, a plurality of theexcitation ports 202 are connected. To each of the excitation ports 202,an excitation light source 201 is connected. Each of the excitationlight emitted from each excitation light source 201 enters the firstcladding of the rare-earth-doped double-clad fiber 210 via the opticalcoupler 203. The excitation lights having entered the first cladding areabsorbed into the rare-earth ions doped in the core, and a populationinversion is formed, to thereby produce stimulated emission. With thestimulated emission, the laser beam propagating through the core isamplified, and is then output via the isolator 206.

In the case of the MO 100 with the MOPA system as shown in FIG. 16, ifthe rare-earth-doped double-clad fiber 210 of the PA 200 is excited, ina state with signal light not being incident from the MO 100, by theexcitation lights emitted from the excitation light sources 201 andreaches a specified population inversion ratio, a parasitic oscillationoccurs, and pulses with a very high peak value are generated. Thepopulation inversion ratio at which a parasitic oscillation occurs isdetermined by the reflectances on the entrance side and the exit side ofthe rare-earth-doped double-clad fiber 210. At some of the populationinversion ratios, pulses with a very high peak value by the parasiticoscillation are emitted from the rare-earth-doped double-clad fiber 210to the optical coupler 203. At this time, there have been problems inthat the core of the rare-earth-doped double-clad fiber 210 is damagedby pulses with a very high peak value and that the pulses reach theexcitation light source 201 and the MO 100 and thereby damage theexcitation light source 201 and the MO 100.

Furthermore, even in the state where pulses are emitted from the MO 100with a cycle that does not produce a parasitic oscillation in the PA200, and the optical fiber laser functions normally, there may be a casewhere reflected light from the outside of the PA 200 output induces aparasitic oscillation while the pulses are input. Normally, theexcitation light source 201 of the PA 200 emits the excitation lightbetween the pulses. Therefore, the rare-earth-doped double-clad fiber210 is in an excited state. Consequently, the ASE is emitted from bothsides of the rare-earth-doped double-clad fiber 210. For example, in thecase where the optical fiber laser is applied to a material machining,the ASE is emitted onto the material to be machined from the opticalfiber laser. At this time, in some of the surface states of the materialto be machined, the light reflected off the surface of the material tobe machined may again enter the optical fiber laser. Then, anoscillation occurs with this reflected light functioning as seeds, andpulses with a very high peak value are emitted from the rare-earth-dopeddouble-clad fiber 210 to the optical coupler 203. This brings about aproblem in that the pulses reach the excitation light sources 201 andthe MO 100 and thereby damage the excitation light source 201 and the MO100.

As described above, in conventional optical fiber lasers, a parasiticoscillation occurs. This has prevented a high population inversion ratiofrom being achieved, and hence, the energy of the pulse capable of beingoutput from the PA 200 has been limited.

To solve these problems, an isolator is inserted into both sides of therare-earth-doped optical fiber, to thereby keep the reflectance low andsuppress the parasitic oscillation, for example, in the method describedin Patent Document 3. Furthermore, a short wavelength pass filter isprovided on the exit end of the excitation light source, to therebyprevent the ASE emitted from the rare-earth-doped optical fiber fromentering again the rare-earth-doped optical fiber after reflection bythe pump laser. That is, in the optical fiber laser described in PatentDocument 3, the reflectances on the entrance side and the exit side ofthe rare-earth-doped optical fiber are suppressed as much as possible,to thereby suppress the parasitic oscillation.

Furthermore, for example, in the method described in Patent Document 4,an optical fiber amplifier is divided into two stages, and an isolatoris provided between the stages. At the previous stage of the opticalfiber amplifier, a gain thereof is kept low, to thereby suppress aparasitic oscillation. On the other hand, at the subsequent stage of theoptical fiber amplifier, a gain thereof is high. However, the ASEemitted from the previous stage of the optical fiber amplifier is alwaysincident therein. Therefore, in the optical fiber laser described inPatent Document 4, an amplification of the ASE is produced, but thisdoes not lead to a parasitic oscillation.

In the optical fiber laser described in Patent Document 5, a fiber Bragggrating (hereinafter, sometimes referred to as FBG) is provided on bothends of a rare-earth-doped double-clad fiber, to thereby construct aresonator. Furthermore, to one of the FBGs, there is connected amulti-mode fiber. An excitation light from an excitation light sourceenters the rare-earth-doped double-clad fiber via the multi-mode fiber.In the optical fiber laser, a core diameter of the multi-mode fiber islarger than that of the rare-earth-doped double-clad fiber. Therefore,the ASE with unnecessary wavelengths that has entered the multi-modefiber without being reflected by the FBG has a low percentage ofreuniting with the core of the rare-earth-doped double-clad fiberthrough reflection. Therefore, a parasitic oscillation is suppressed.Even if a parasitic oscillation occurs, the generated pulses enter themulti-mode fiber at first. Therefore, even the ASE is collected on theexcitation light source via a lens, its spot diameter becomes large.Consequently, the excitation light source is unlikely to be damaged.

However, in the method described in Patent Document 3, the suppressionof the reflectance is, in actuality, approximately 0.001% at best.Therefore, in a comparatively high-output optical fiber laser that emitstens of watts or more laser beam, there is a possibility that aparasitic oscillation will occur with this slight reflection or Rayleighscattering in the fiber functioning as seeds, no matter much thereflection is suppressed. Furthermore, as for reflected light from theoutside that is produced after the laser emission (reflected light onthe surface of the material to be machined), the intensity of thereflected light is attenuated by the isolator. However, it is impossibleto completely suppress the reflected light. This leads to a possibilityof inducing a parasitic oscillation with the slightly remainingreflected light functioning as seeds.

In the method described in Patent Document 4, a high-gain amplifier isprovided on the exit side. Therefore, there is a possibility of inducinga parasitic oscillation because reflected light from the outside entersthe high-gain amplifier at the beginning To address this, the use oflow-gain amplifiers in multiple stages instead of a high-gain amplifiercan be conceived. However, in this case, the higher the output is, thegreater the number of the stages is. This results in a complexconfiguration, and hence, lowers efficiency.

In the method described in Patent Document 5, light with wavelengths notreflected by the FBG has a low percentage of reuniting with the core ofthe rare-earth-doped double-clad fiber after reflection off the end faceof the multi-mode fiber. However, the light does not unite with the coreat all. Therefore, the higher the gain of an optical fiber amplifierbecomes, the higher the percentage of reuniting with the core is. Thisleads to a possibility of a parasitic oscillation. Furthermore, as forreflected light from the outside, light with the same wavelength as thereflected wavelength of the FBG is reflected by the FBG. However, lightoutside the wavelength passes through the FBG and enters therare-earth-doped double-clad fiber. Therefore, there is a possibilitythat a parasitic oscillation is induced by the light which has enteredthe rare-earth-doped double-clad fiber.

-   Patent Document 1: Japanese Patent No. 2977053-   Patent Document 2: U.S. Pat. No. 5,864,644-   Patent Document 3: Japanese Patent Publication, First Publication    No. H05-136498-   Patent Document 4: Japanese Patent No. 2653936-   Patent Document 5: Japanese Patent Publication, First Publication    No. H10-56227

SUMMARY OF THE INVENTION

The present invention has been achieved in view of the abovecircumstances, and has an object to provide an optical fiber laser inwhich a parasitic oscillation is suppressed and a pulse with high energyis capable of being stably emitted.

To solve the above problems and achieve the object, the presentinvention adopts the followings.

(1) An optical fiber laser of the present invention is an optical fiberlaser including: a master oscillator which is a laser oscillatorproducing a seed beam; and a power amplifier which is an opticalamplifier connected to a subsequent stage of the master oscillator andamplifying and outputting a laser beam emitted from the masteroscillator, in which the power amplifier includes: a plurality ofexcitation light sources; excitation ports each of which is connected tothe excitation light sources and which an excitation light emitted fromeach of the excitation light source enters; a signal port which a laserbeam emitted from the master oscillator enters; an optical coupler withan exit port that outputs the excitation light from the excitation portstogether with the laser beam from the signal port; and an optical fiberconnected to the exit port, in which the optical fiber is a photonicbandgap fiber, and in which the optical fiber has a loss wavelengthcharacteristic in that a photonic bandgap region is narrower than a gainwavelength band in a graph with an axis of abscissa representingwavelength and an axis of ordinate representing loss amount.

(2) It is preferable that the optical fiber include: a core portion madeof a solid material doped with a rare-earth element; a first claddingprovided around the core portion; and a periodic structure portion inwhich a multitude of high refractive index portions with a refractiveindex higher than that of the first cladding are arranged in a periodicstructure, the periodic structure portion being provided in a vicinityof the core portion in the first cladding.

(3) It is preferable that the maximum relative index difference of thehigh refractive index portion be 2% to 3% with respect to the firstcladding.

(4) It is preferable that the core portion be higher than the periodicstructure portion in electric field distribution of light in thephotonic bandgap region and that the periodic structure portion behigher than the core portion in electric field distribution of lightoutside the photonic bandgap region.

(5) It is preferable that at least germanium be included in the highrefractive index portion.

(6) An optical fiber laser of the present invention is an optical fiberlaser including: a master oscillator as a laser oscillator for producinga seed beam; and a power amplifier as an optical amplifier that isconnected at a subsequent stage of the master oscillator for amplifyingand outputting a laser beam emitted from the master oscillator, in whichthe master oscillator includes: an excitation light source; a WDMcoupler that is connected to the excitation light source for combiningan excitation light from the excitation light source with a laser beam;an optical fiber connected to the WDM coupler; an output couplerconnected to the optical fiber; and an isolator, in which the WDMcoupler, the optical fiber, the output coupler, and the isolator areconnected in this order in a ring, and in which the optical fiber has aloss wavelength characteristic in that a photonic bandgap region isnarrower than a gain wavelength band in a graph with an axis of abscissarepresenting a wavelength and an axis of ordinate representing a lossamount.

(7) An optical fiber laser of the present invention including: a masteroscillator as a laser oscillator for producing a seed beam; and a poweramplifier as an optical amplifier that is connected at a subsequentstage of the master oscillator for amplifying and outputting a laserbeam emitted from the master oscillator, in which the power amplifier isthe power amplifier according to the above (1), and the masteroscillator is the master oscillator according to the above (6).

According to the optical fiber laser as set forth in the above (1), itbecomes possible to efficiently eliminate ASE with unnecessarywavelengths that are a cause of a parasitic oscillation. Consequently,it is possible to lengthen the time until an occurrence of a parasiticoscillation. Therefore, the optical fiber laser can store more energythan conventional optical fiber lasers. As a result, when pulsed lightis amplified, high-gain amplification can be performed, and hence, apulsed output with high energy that conventional optical fiber lasershave not been capable of outputting is available.

Furthermore, in conditions under which a parasitic oscillation hasconventionally occurred, a parasitic oscillation does not occur in theoptical fiber laser of the present invention. Therefore, it is possibleto suppress damage done to the parts of the fiber laser due to theparasitic oscillation. Furthermore, in an optical part such as a filteror an isolator with low power resistance that is conventionally used,these optical parts are heated to a high temperature because ofeliminating unnecessary and locally high intense light. Therefore, ithas been required a cooling apparatus, or has brought aboutdeterioration in the characteristics of the part. In the optical fiberlaser of the present invention, unnecessary light is eliminated withrespect to a wavelength distribution. This prevents unnecessary lightbecoming highly intense. Therefore, the unnecessary light has littleeffect on optical parts, facilitating cooling. Therefore, it is possibleto provide an optical fiber laser which can be used for a long timestable. In addition, in the optical fiber laser according to the above(6), similar operational advantages are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram schematically showing an optical fiber laseraccording to a first embodiment of the present invention.

FIG. 1B is a diagram showing a loss wavelength characteristic of anoptical fiber used in the optical fiber laser according to theembodiment.

FIG. 2A is a cross-sectional view schematically showing the opticalfiber used in the optical fiber laser according to the embodiment.

FIG. 2B is a diagram showing a refractive index profile of the opticalfiber used in the optical fiber laser according to the embodiment.

FIG. 2C is a diagram schematically showing the electrical fielddistribution of the optical fiber used in the optical fiber laseraccording to the embodiment.

FIG. 3A is a cross-sectional view schematically showing an optical fiberused in an optical fiber laser according to a second embodiment of thepresent invention.

FIG. 3B is a diagram showing a refractive index profile of the opticalfiber used in the optical fiber laser according to the embodiment.

FIG. 3C is a diagram schematically showing the electric fielddistribution of the optical fiber used in the optical fiber laseraccording to the embodiment.

FIG. 4A is a cross-sectional view schematically showing an optical fiberused in an optical fiber laser according to a third embodiment of thepresent invention.

FIG. 4B is a diagram showing a refractive index profile of the opticalfiber used in the optical fiber laser according to the embodiment.

FIG. 4C is a diagram schematically showing an electric fielddistribution of the optical fiber used in the optical fiber laseraccording to the embodiment.

FIG. 5A is a cross-sectional view schematically showing an optical fiberused in an optical fiber laser according to a fourth embodiment of thepresent invention.

FIG. 5B is a diagram showing a refractive index profile in the A-A′direction of the optical fiber used in the optical fiber laser accordingto the embodiment.

FIG. 5C is a diagram schematically showing a refractive index profile inthe B-B′ direction of the optical fiber used in the optical fiber laseraccording to the embodiment.

FIG. 5D is a diagram showing the electric field distribution in the A-A′direction of the optical fiber used in the optical fiber laser accordingto the embodiment.

FIG. 5E is a diagram schematically showing an electric fielddistribution in the B-B′ direction of the optical fiber used in theoptical fiber laser according to the embodiment.

FIG. 6A is an electron micrograph of a cross-section of an optical fiberused in an optical fiber laser according to a fifth embodiment of thepresent invention.

FIG. 6B is a diagram showing a refractive index profiles of the opticalfiber used in the optical fiber laser according to the embodiment.

FIG. 6C is a diagram schematically showing the electric fielddistribution in the X-axis direction of the optical fiber used in theoptical fiber laser according to the embodiment.

FIG. 6D is a diagram schematically showing the electric fielddistribution in the Y-axis direction of the optical fiber used in theoptical fiber laser according to the embodiment.

FIG. 7 is a diagram showing an optical fiber laser according to a sixthembodiment of the present invention.

FIG. 8 is a diagram in which an optical fiber laser of Example 1 is usedto observe the time from the entry of an excitation light to theoccurrence of a parasitic oscillation.

FIG. 9 is a diagram in which an optical fiber laser of Example 2 is usedto observe the time from the entry of an excitation light to theoccurrence of a parasitic oscillation.

FIG. 10 is a diagram in which an optical fiber laser of ComparativeExample is used to observe the time from the entry of an excitationlight to the occurrence of a parasitic oscillation.

FIG. 11 is a diagram schematically showing an apparatus used when atransmission spectrum of an optical fiber used in Example 4 is measured.

FIG. 12 is a diagram showing the transmission spectrum of the opticalfiber used in Example 4.

FIG. 13A is a diagram when an ASE spectrum of the optical fiber used inExample 4 is measured.

FIG. 13B is a diagram when an ASE spectrum of the optical fiber used inExample 4 is measured.

FIG. 14 is a schematic diagram of a representative optical fiber laserwith the MOPA system.

FIG. 15 is a diagram schematically showing a general MO.

FIG. 16 is a diagram schematically showing a general PA and a generaloptical fiber laser.

DESCRIPTION OF THE REFERENCE SYMBOLS

10 (10A, 10B, 10C, 10D, 10E): optical fiber

11, 21, 31, 41, 51: core portion

12, 22, 32, 42, 52: periodic structure portion

13, 23, 33, 43, 53: first cladding

14, 24, 32, 44, 54: high refractive index portion

100: master oscillator (MO)

101: excitation light source

102: WDM coupler

104: isolator

105: output coupler

107: optical switch element

200: power amplifier (PA)

201: excitation light source

202: excitation port

203: optical coupler

205: exit port

206: isolator

316: interstage isolator

DETAILED DESCRIPTION OF THE INVENTION

<First Embodiment>

Hereunder is a detailed description of the present invention withreference to the drawings. However, the present invention is not limitedto this. Various modifications can be made without departing from thesprit or scope of the present invention.

FIG. 1A is a configuration diagram schematically showing an opticalfiber laser 50 according to a first embodiment of the present invention.FIG. 1B shows a loss wavelength characteristic of an optical fiber 10used in the optical fiber laser 50 of the present embodiment.

Similarly to the optical fiber laser shown in FIG. 14, the optical fiberlaser of the present embodiment is an optical fiber laser 50 with theMOPA system in which a master oscillator (hereinafter, sometimesreferred to as MO) 100 of a laser oscillator for producing a seed beamis connected with a power amplifier (hereinafter, sometimes referred toas PA) 200 that amplifies and outputs the laser beam emitted from the MO100. The power amplifier is at the subsequent stage of the MO. As the MO100, for example a fiber ring laser 100 shown in FIG. 15 can be used.The output of the MO 100 is connected to the PA 200 via an interstageisolator 316. The PA 200 includes: a plurality of excitation lightsources 201; excitation ports 202 each of which is connected to theexcitation light sources 201, and each of which an excitation lightemitted from each excitation light source 201 enters; a signal port 204which a laser beam emitted from the master oscillator 100 enters; anoptical coupler 203 with an exit port 205 that outputs the excitationlights having entered from the excitation ports 202 together with thelaser beam having entered from the signal port 204; and an optical fiber10 connected to the exit port 205. The laser beam emitted from the MO100 enters the PA 200 via the signal port 204, and is then incident intothe core of the optical fiber 10 via the optical coupler 203. Here, theoptical fiber 10 is provided with a core and a cladding that surroundsthe core. The core is doped with rare-earth ions. The configuration ofthe optical fiber 10 will be described in detail later. On the otherhand, the excitation lights emitted form the excitation light sources201 enter a first cladding of the optical fiber 10 via the opticalcoupler 203. The excitation lights having entered the first cladding ofthe optical fiber 10 are absorbed into the rare-earth ions doped in thecore, thus forming a population inversion. This produces stimulatedemission. With the stimulated emission, the laser beam propagatingthrough the core is amplified. The amplified laser beam is output viathe isolator 206. That is, the laser beam that has been output from theMO 100 enters the PA 200 via the interstage isolator 316. The laser beamis amplified by the PA 200, and is then output.

Hereunder is a detailed description thereof.

For the optical coupler 203, a conventional, known optical coupler suchas disclosed, for example, in Patent Document 2 is used. On one side,the optical coupler 203 has a plurality of excitation ports 202 made ofa multi-mode optical fiber, and a signal port 204 made of a single-modefiber. On the other side, the optical coupler 203 has an exit port 205that emits excitation lights having entered from the excitation ports202 together with a laser beam having entered from the signal port 204.

As an excitation light source 201, a laser diode (LD) or the like isfavorably used. However, the excitation light source 201 is not limitedto this.

The optical fiber 10 is a photonic bandgap fiber, and has a losscharacteristic shown in FIG. 1B. In the graph of FIG. 1B, the axis ofabscissas represents the wavelength of light, and the axis of ordinaterepresents the loss amount of light. In the loss wavelengthcharacteristic of the optical fiber 10 according to the presentembodiment, the photonic bandgap is present in the gain wavelength bandof the optical amplification by the rare-earth ions doped in the core.Furthermore, the wavelength band of the photonic bandgap is narrowerthan the gain wavelength band. In addition to the relationship betweenthe wavelength bands, the photonic bandgap has its configurationdetermined as follows so as to include the oscillation wavelength of theoptical fiber laser 50. That is, in a wide range of wavelength bands ofspontaneous emission that is produced when the rare-earth ions doped inthe core portion is excited, the photonic bandgap is formed only in theoscillation wavelength band to be guided. Therefore, light with theoscillation wavelength propagates while confined in the core portion ofthe optical fiber 10. On the other hand, spontaneous emission in thewavelength band outside the oscillation wavelength is released into thecladding without being confined in the core portion. That is, thespontaneous emission in the wavelength band outside the oscillationwavelength band that functions as seeds of a parasitic oscillation isreleased into the cladding from the core portion. Therefore, it ispossible to suppress a parasitic oscillation.

A parasitic oscillation is likely to occur especially at a wavelength atwhich the gain of the rare-earth-doped optical fiber is maximum. If theloss of the resonator has a wavelength dependence, a parasiticoscillation occurs in a wavelength region in which the differencebetween the gain of the rare-earth-doped optical fiber and the loss ofthe resonator (gain-loss) is maximized. Therefore, when such awavelength band is set to be outside the photonic bandgap region asshown in FIG. 1B, the effect of suppressing a parasitic oscillationbecomes large.

An optical fiber 10A (10) for use in the optical fiber laser 50 of thepresent embodiment has a cross-sectional configuration as shown, forexample, in FIG. 2A. The optical fiber 10A roughly includes: a coreportion 11 made of a solid material doped with a rare-earth element; afirst cladding 13 provided around the core portion 11; and a periodicstructure portion 12 provided in the vicinity of the core portion 11 ofthe first cladding 13, in which a multitude of high refractive indexportions 14 with a refractive index higher than that of the firstcladding 13 are arranged in a periodic structure. Although not shown inthe figure, around the outer periphery of the first cladding 13, theremay be provided a fluorine-based ultraviolet-curing resin layer in whicha relative index difference from pure silica is a negative value (forexample, approximately −5%).

Furthermore, the optical fiber 10A has a completely solid configurationwith no holes. Therefore, when the optical fiber 10A of the presentembodiment is fusion-spliced with another optical fiber, holes willnever become flat. Therefore, it is possible to fusion-splice theoptical fiber 10A at a low optical loss.

Each of the core portion 11, the periodic structure portion 12, and thefirst cladding 13 of the optical fiber 10A is made of pure silica glassor silica-based glass formed of pure silica glass doped with a dopantfor adjusting a refractive index such as fluorine or germanium oxide. Inthe optical fiber 10 of the present embodiment, the materials for eachportion are not limited to the examples illustrated in the presentembodiment.

The core portion 11 is formed of pure silica doped with rare-earth ions.It is preferable that the core portion 11 has a refractive indexequivalent to that of pure silica. Types of rare-earth ion to be dopedinclude, for example, ytterbium (Yb), erbium (Er), thulium (Tm),neodymium (Nd), and praseodymium (Pr). These rare-earth ions may be usedalone or mixed in a desired ratio.

In the vicinity of the core portion 11 in the first cladding 13, thereare arranged a multitude of high refractive regions (high refractiveindex portions 14) which each have a small circular shaped cross-sectionand are doped with germanium (Ge) and the like. The high refractiveindex portions 14 are arranged in a periodic structure in a triangularlattice, to thereby form a periodic structure portion 12. In the presentspecification, a triangular lattice refers to a configuration in which afirst high refractive index portion 14 a, a second high refractive indexportion 14 b adjacent to the first high refractive index portion 14 a,and a third high refractive index portion 14 c adjacent to both of thefirst high refractive index portion 14 a and the second high refractiveindex portion 14 b form an equilateral triangle.

With the adjustment of the diameter and period of the high refractiveindex portions 14, the adjustment of the distance between the highrefractive index portions 14, and the adjustment of the relative indexdifference between the high refractive index portion 14 and pure silicaglass, it is possible to form a photonic band in a desired wavelengthband. For example, the high refractive index portion 14 has a diameterof 3 μm to 5 μm, the distance between the high refractive index portions14 is 5 μm to 10 μm, the high refractive index portions 14 have a periodof 3 layers to 6 layers, and the maximum relative index differencebetween the high refractive index portion 14 and the first cladding is2% to 3%.

In the optical fiber 10A of the present embodiment, the region of thefirst cladding 13 in which the periodic structure portion 12 is arrangedoccupies more area than the region of the first cladding 15 in which theperiodic structure portion 12 is not arranged.

Thus, in the optical fiber 10A used in the present embodiment, the coreportion 11 with a refractive index equal to that of pure silica isarranged at the center, and the periodic structure portion 12 isarranged therearound. The periodic structure portion 12, which is dopedwith Ge and the like, has a refractive index higher than that of thecore portion 11. Therefore, the refractive index profile of the opticalfiber 10A is illustrated as shown in FIG. 2B. As shown in FIG. 2B, thecore portion 11 and the first cladding 13 are equivalent in refractiveindex, and the refractive index of the periodic structure portion 12(the refractive index of the high refractive index portions 14) ishigher than that of the core portion 11 and the first cladding 13.

When light enters the optical fiber 10A, light in the wavelength regionof the photonic bandgap is not capable of being guided through theperiodic structure portion 12 in which the high refractive indexportions 14 are arranged. Therefore, the light in the wavelength regionof the photonic bandgap is confined in the lower refractive indexregion, that is, in the core portion 11. As a result, in the light inthe wavelength region of the photonic bandgap, the electric fieldconcentrates in the core portion 11 as shown in the upper portion ofFIG. 2C, and is guided through the core portion 11. That is, withrespect to light in the wavelength region of the photonic bandgap, thelower refractive region functions as a core portion, and the higherrefractive region (the periodic structure portion 12 made of the highrefractive index portions 14) functions as a cladding.

On the other hand, light in the wavelength region outside the photonicbandgap can be guided not only through the core portion 11, but alsothrough the periodic structure portion 12. At this time, its electricfield distribution is widely different from the electric fielddistribution of the wavelengths in the photonic bandgap region. As shownin the lower portion of FIG. 2C, most of the electric field is in theperiodic structure portion 12. Thus, between the wavelength region ofthe photonic bandgap and the other wavelength regions, there is a largedifference in electric field distribution when light is guided throughthe fiber. Light in the wavelength region outside that of the photonicbandgap is spread all over the optical fiber 10A from the core portion11 and is then radiated. Therefore, the spontaneous emission produced bythe excitation of the rare-earth ions is released from the core portion11 to the cladding before it is turned to ASE light.

As described above, in the optical fiber laser 50 of the presentembodiment, the optical fiber 10A is used, to thereby make it possibleto efficiently eliminate ASE with unnecessary wavelengths that is acause of a parasitic oscillation, and to lengthen the time until anoccurrence of a parasitic oscillation. Therefore, the optical fiberlaser 50 can store more energy than conventional optical fiber lasers.Consequently, when pulsed light is amplified, amplification with a highgain can be performed, and hence, a pulsed output with high energy thatconventional optical fiber lasers have not been able to output isavailable. Furthermore, in conditions under which a parasiticoscillation has conventionally occurred, a parasitic oscillation doesnot occur in the present embodiment. Therefore, it is possible tosuppress damage occurring in the parts of the fiber laser. Furthermore,unnecessary light is eliminated with respect to a wavelengthdistribution. This prevents the unnecessary light from becoming highlyintense. Therefore, the unnecessary light has a little effect on theoptical parts, facilitating the cooling of the parts constituting theoptical fiber laser. Consequently, an optical fiber laser which can beused for a long time stable is obtained.

<Second Embodiment>

FIG. 3A is a cross-sectional view schematically showing an optical fiber10B (10) mounted in an optical fiber laser according to a secondembodiment. FIG. 3B is a diagram showing a refractive index profile ofthe optical fiber 10B used in the present embodiment. FIG. 3C is adiagram showing an electric field distribution of the optical fiber 10Bused in the present embodiment.

The optical fiber laser of the present invention is different from theoptical fiber laser of the first embodiment in that the optical fiber10B is used that has a cross-section shown in FIG. 3A, and also has therefractive index profile shown in FIG. 3B and the electric fielddistribution shown in FIG. 3C.

In the optical fiber 10B used in the present embodiment, the first layer(the innermost layer) of the high refractive index portion in theperiodic structure of the optical fiber 10A of the first embodiment isremoved, and a region 21 a in which pure silica is arranged is formed.In the region 21 a, there is arranged a core portion 21 doped with Yband additionally with Al, Ge or the like, and hence, with a refractiveindex higher than that of the pure silica therearound by approximately0.1% to 0.5%. Therefore, the refractive index profile of the opticalfiber 10B of the present embodiment has a portion with a higherrefractive index (core portion 21) also in the region 21 a.

In the optical fiber 10B of the present embodiment, light with awavelength in the photonic bandgap region is not capable of being guidedthrough a region with a higher refractive index in which high refractiveindex portions 24 are arranged (a periodic structure portion 22),similarly to the optical fiber 10A of the first embodiment. Therefore,the light is guided inner than the periodic structure portion 22. In theoptical fiber 10B of the present embodiment, the core portion 21 with arefractive index higher than that of pure silica is formed at thecenter. Therefore, in the light with wavelengths in the photonic bandgapregion, the electric field concentrates in the core portion 21 moreintensely than that of the optical fiber 10A used in the firstembodiment, as shown in the upper portion of FIG. 3C.

On the other hand, as for light with wavelengths outside the photonicbandgap region, most of the electric field is present in the periodicstructure portion 22. However, the core portion 21 of the presentembodiment has a refractive index higher than that of the pure silicatherearound. Consequently, as shown in the lower portion of FIG. 3C, anelectric field is slightly present also in the core portion 21 of theoptical fiber 10B. Therefore, light with wavelengths outside thephotonic bandgap region is capable of being guided through the opticalfiber 10B.

The optical fiber 10B of the present embodiment is formed by rotating anoptical fiber raw material while the fiber is spun. Therefore, theperiodic structure portion 22 is spirally twisted in the lengthdirection of the fiber. Consequently, bending is applied substantiallyin the length direction of the optical fiber raw material. As a result,in the optical fiber 10B, an optical loss is generated due to macrobendsor microbends. In the optical fiber 10B used in the present embodiment,the optical loss is inflicted on light which is guided through the coreportion 21 in the wavelength region outside the photonic bandgap, tothereby eliminate the light.

On the other hand, the core portion 21 is at the center of the opticalfiber 10B. Therefore, the twist does not affect itself in the shape ofthe core portion 21. As a result, in light with a wavelength in thephotonic bandgap region propagating through the core portion 21, theoptical loss due to macrobent or microbent is not generated, and hence,the light is not eliminated from the core portion 11.

The pitch of the spiral may be appropriately adjusted so as to becapable of inflicting loss on light in a desired wavelength region.

As described above, by use of the optical fiber 10B, it is possible toeffectively eliminate light with wavelengths outside the photonicbandgap. As a result, in the optical fiber laser using the optical fiber10B, ASE with unnecessary wavelengths that is a cause of a parasiticoscillation is eliminated, making it possible to lengthen the time untilan occurrence of a parasitic oscillation. Therefore, advantages similarto those of the optical fiber laser 50 of the first embodiment areobtained. Especially in the optical fiber 10B of the present embodiment,the core portion 21 has a refractive index higher than that of theoptical fiber 10A of the first embodiment. Therefore, the electric fieldof light with the wavelengths in the photonic bandgap region is presentin the core portion 21 more intensely than in the case of the opticalfiber 10A of the first embodiment. As a result, in the optical fiberlaser of the present embodiment, the laser beam propagating through thecore portion 21 can be further amplified. Hence, it is possible toobtain a pulsed output with energy higher than that of the optical fiberlaser of the first embodiment.

In the optical fiber 10B used in the present embodiment, a loss isinflicted by macrobends or microbends on light, guided through the coreportion 21, with wavelengths outside the photonic bandgap. However, adopant that can absorb or scatter light with a fluorescence wavelengthof Yb may be doped in the periodic structure portion 22, to therebyinflict a loss on light with wavelengths outside the photonic bandgap.

If return losses on the entrance side and the exit side of the opticalfiber 10B are larger than the gain obtained in the optical fiber 10B, itis possible to suppress parasitic oscillation. When a light guidedthrough the optical fiber 10B exits from the optical fiber 10B, thelight suffers from a loss twice: when it exits from the optical fiber10B; and when it enters the optical fiber 10B. Therefore, when lightexits from or is incident to the optical fiber 10B, it is possible tosuppress a parasitic oscillation if the loss received by the light withwavelengths outside the photonic bandgap is more than half of the gainobtained when the light is guided through the optical fiber 10B.

<Third Embodiment>

FIG. 4A is a cross-sectional view schematically showing an optical fiber10C (10) mounted in an optical fiber laser according to a thirdembodiment. FIG. 4B is a diagram showing a refractive index profile ofthe optical fiber 10C used in the present embodiment. FIG. 4C is adiagram showing an electric field distribution of the optical fiber 10Cused in the present embodiment.

The optical fiber laser of the present embodiment is different from theoptical fiber laser of the first embodiment in that the optical fiber10C is used that has a cross-section shown in FIG. 4A, and also has therefractive index profile shown in FIG. 4B and the electric fielddistribution shown in FIG. 4C.

In the optical fiber 10C used in the present embodiment, the inner firstand second layers (the portion in the vicinity of the core) of theperiodic structure portion 12 of the first embodiment is removed, and aregion 31 a made of pure silica is formed. In the region 31 a, there isformed a core portion 31 doped with Yb and additionally with Al, Ge orthe like, and hence, with a refractive index higher than that of thepure silica therearound by approximately 0.1% to 0.5%. Therefore, therefractive index profile of the optical fiber 10C of the presentembodiment has a portion with a higher refractive index also in a coreportion 31 of the region 31 a.

As for a periodic structure portion 32, the distance between highrefractive index portions 34 is narrower by approximately 10% to 20%than that of the periodic structure portion 12 of the first embodiment.The high refractive index portions 34 are packed in the vicinity of thecore portion 31. The diameter of the high refractive index portion 34 islarger than that of the first embodiment by approximately 30% to 40%.The relative index difference between the high refractive index portion34 and pure silica glass is equivalent to the case of the optical fibersin the first embodiment and in the second embodiment. The periodicstructure portion 32 of the optical fiber 10 of the present embodimenthas layers less than that of the first embodiment, for example,approximately three layers. The first cladding 33 has a diameter similarto that of the first embodiment. Therefore, a region 35 of the firstcladding 33 in which the periodic structure portion 32 is not arrangedis wider than that of the first embodiment.

Around the outer periphery of the first cladding 33, there is arranged afluorine-based ultraviolet-curing resin layer 36 in which a relativeindex difference from pure silica is a negative value (for example,approximately −5%).

The optical fiber 10C used in the present embodiment has a double-cladconstruction in which light is guided in multimode with a first cladding33 made of pure silica glass used as a second core and a fluorine-basedultraviolet-curing resin 36 used as a second cladding.

The refractive index profile and the electric field distribution aresimilar to those of the second embodiment, which are as shownrespectively in FIG. 4B and FIG. 4C. That is, in the light in thephotonic bandgap region, the electric field concentrates in the coreportion 31 more intensely than that of the optical fiber 10A used in thefirst embodiment, as shown in the upper portion of FIG. 4C. Light withwavelengths outside the photonic bandgap is capable of being guidedthrough the optical fiber 10C because an electric field is present notonly in the periodic structure portion 32 but also slightly in the coreportion 31 of the optical fiber 10C. In the optical fiber 10C used inthe present embodiment, light outside the photonic bandgap region iseliminated by macrobends or microbends, similarly to the optical fiber10B used in the second embodiment. As for the core portion 31, in lightwith wavelengths in the photonic bandgap region, an optical loss due tomacrobends or microbends is not generated, similarly to the opticalfiber 10B of the second embodiment. Hence, the light is not eliminatedfrom the core portion 31.

As described above, by use of the optical fiber 10C, it is possible toeffectively eliminate light with wavelengths outside the photonicbandgap region. As a result, in the optical fiber laser using theoptical fiber 10C, ASE in unnecessary wavelengths that is a cause of aparasitic oscillation is eliminated, making it possible to lengthen thetime until an occurrence of a parasitic oscillation. Therefore,advantages similar to those of the optical fiber laser of the secondembodiment are obtained.

<Fourth Embodiment>

FIG. 5A is a cross-sectional view schematically showing an optical fiber10D (10) mounted in an optical fiber laser according to a fourthembodiment. FIG. 5B is a refractive index profile in the A-A′ directionof the optical fiber 10D. FIG. 5C is a refractive index profile in theB-B′ direction of the optical fiber 10D. FIG. 5D is an electric fielddistribution in the A-A′ direction of the optical fiber 10D. FIG. 5E isan electric field distribution in the B-B′ direction of the opticalfiber 10D.

The optical fiber laser of the present embodiment is different from theoptical fiber laser of the first embodiment in that an optical fiber 10Dis used that has a cross-section shown in FIG. 5A, and also has therefractive index profiles shown in FIGS. 5B, 5C and the electric fielddistributions shown in FIGS. 5D, 5E.

In the optical fiber 10D used in the present embodiment, the relativeindex difference from the pure silica glass of the core portion 41, thedistance between the high refractive index portions 44, the diameter ofthe high refractive index portion 44, the relative index difference ofthe high refractive index portion 44, and the like are similar to thoseof the optical fiber 10C used in the third embodiment. The optical fiber10D of the present embodiment has more layers of high refractive indexportions 44 than the optical fiber 10C of the third embodiment. Inaddition, along linear regions 47 from the core portion 41 to an outerperiphery of the optical fiber, the high refractive index portions 44are not arranged.

The refractive index profiles and the electric field distributions areas shown in FIGS. 5B to 5E. That is, in light with a wavelength in thephotonic bandgap region, the electric field concentrates in the coreportion 41 more intensely than the case of the optical fiber 10A used inthe first embodiment, as shown in the upper portions of FIG. 5D and FIG.5E.

Light with the wavelengths outside the photonic bandgap region iscapable of being guided through the optical fiber 10D because anelectric field is present not only in a periodic structure portion 42but also slightly in the core portion 41 of the optical fiber 10D asshown in lower portions of FIG. 5D and FIG. 5E.

In the optical fiber 10D used in the present embodiment, light outsidethe photonic bandgap region that is guided through the core portion 31is eliminated due to macrobends or microbends, similarly to the opticalfiber 10B used in the second embodiment. As for the core portion 41, inlight with wavelengths in the photonic bandgap region, an optical lossdue to macrobends or microbends is not generated similarly to theoptical fiber 10B of the second embodiment. Hence, the light is noteliminated from the core portion 41.

Especially, in the optical fiber 10D used in the present embodiment,more layers of the high refractive index portions 44 are arranged thanthose in the optical fiber 10C used in the third embodiment. Therefore,a greater amount of light outside the photonic bandgap region is presentin the periodic structure portion 42. This can further enhance asuppression effect on parasitic oscillation. In addition, even if aconfinement effect on light by the photonic bandgap becomes strongerwith an increased number of layers of the high refractive index portions44, high-order-mode light is radiated through the regions 47 in whichthe high refractive index portions 44 are not arranged. This allows astable operation in basic mode.

Therefore, the optical fiber laser using the optical fiber 10D cansuppress the occurrence of a parasitic oscillation more effectively thanthe optical fiber laser of the third embodiment, allowing a stableoperation in basic mode.

<Fifth Embodiment>

FIG. 6A is an electron micrograph of a cross-section of an optical fiber10E (10) mounted in an optical fiber laser according to a fifthembodiment. FIG. 6B is a diagram showing refractive index profiles ofthe optical fiber 10E. FIG. 6C is a diagram showing an electric fielddistribution in the X-axis direction of the optical fiber 10E. FIG. 6Dis a diagram showing an electric field distribution in the Y-axisdirection of the optical fiber 10E.

The optical fiber laser of the present embodiment is different from theoptical fiber laser of the first embodiment in that the optical fiber10E that has a cross-section shown in FIG. 6A, and that has therefractive index profiles shown in FIG. 6B and the electric fielddistributions shown in FIGS. 6C, 6D is used.

In the optical fiber 10E used in the present embodiment, the highrefractive index portions 64 are arranged only in a single line from acore portion 51 to an outer periphery of the optical fiber. The highrefractive index portion 54 is similar to that of the optical fiber 10Aused in the first embodiment. Therefore, the optical fiber 10E hasrefractive index profiles as shown in FIG. 6B. The upper portion of FIG.6B is a refractive index profile in the X-axis direction shown in FIG.6A. The lower portion of FIG. 6B is a refractive index profile in theY-axis direction shown in FIG. 6A.

When light enters the optical fiber 10E, light in the wavelength regionof the photonic bandgap is not capable of being guided through the highrefractive index portion due to a photonic bandgap formed by pure silicaarranged in the X-axis direction and by a periodic structure made ofGe-doped high refractive index portions. Therefore, the light isconfined in a core region 51. As for the Y-axis direction, the light isconfined in the core region 51 due to a refractive index differencebetween pure silica and a fluorine- (F—) doped low refractive indexportion, to thereby be guided the optical fiber 10E.

On the other hand, light outside the wavelength region of the photonicbandgap can be confined in the core region 51 in the Y-axis directiondue to a refractive index difference between pure silica and a F-dopedlow refractive index portion, similarly to light in the wavelengthregion of the photonic bandgap. However, in the X-axis direction, thelight is guided with most of the electric field distribution not beingin the core region 51 but in the high refractive index portion due tothe periodic structure.

As described above, by use of the optical fiber 10E, it is possible toeffectively eliminate light with wavelengths outside the photonicbandgap. As a result, in the optical fiber laser using the optical fiber10E, ASE in unnecessary wavelengths that is a cause of parasiticoscillation is eliminated, making it possible to lengthen the time untilan occurrence of a parasitic oscillation long. Therefore, advantagessimilar to those of the optical fiber laser 50 of the aforementionedfirst embodiment are obtained. Furthermore, in the optical fiber 10E,there is a significant difference in refractive index structure betweenin the X-axis direction and in the Y-axis direction. This provides thecore with birefringence. With the birefringence, light guided throughthe core has different refractive indices due to its polarizedcomponents, and hence, shows different optical characteristics.Especially, a difference arises in the loss characteristic that isgenerated when the optical fiber is bent. A loss due to bending is morelikely to be generated in the polarized component in the X-axisdirection than in the polarized component in the Y-axis direction.Therefore, the optical fiber is bent around a diameter to produce a bendloss only in the Y-axis direction according to the oscillationwavelength of the laser, to thereby make it possible to selectivelyamplify and output the polarized component in the X-axis direction. Thatis, only an application of bending on the optical fiber makes itpossible to output a laser beam with a single polarization wave withoutdecreasing efficiency.

In addition, as another embodiment, it is also possible to offer a lossin the gain wavelength band, similarly to in the present invention, in arare-earth-doped optical fiber which is constructed as follows. Arare-earth-doped optical fiber provided with the FBG in the core portionall over its length. A rare-earth-doped optical fiber is wound around apredetermined diameter, to thereby offer a loss to a long wavelengthside of the signal wavelength. Alternatively, a cutoff wavelength of therare-earth-doped optical fiber is provided on a slightly shortwavelength side of the signal wavelength, and bending is applied to therare-earth-doped optical fiber, to thereby offer a loss to the shortwavelength side of the signal wavelength.

<Sixth Embodiment>

FIG. 7 is a diagram schematically showing an optical fiber laser 100according to a sixth embodiment.

In the present embodiment, each of the optical fibers 10A to 10E usedrespectively in the aforementioned first to fifth embodiments isapplicable to an optical fiber of an MO.

That is, the MO is roughly made of: a WDM coupler 102 that is connectedto an excitation light source 101 and that combines an excitation lightfrom the excitation light source 101 with a laser beam; an optical fiber10; an output coupler; and an isolator 104. These constituent elementsare connected in this order in a ring. In the MO, the optical fiber 10is any of the aforementioned photonic bandgap fibers 10A to 10E.

With an application of any of the above-mentioned optical fibers 10A to10E to the MO, it is possible to suppress a parasitic oscillation, andto obtain an MO with which a high output is available, similarly to thecase where the application is made to the PA.

The application of the optical fiber 10 used in any of theaforementioned first to fifth embodiments is not limited to only one ofthe PA and the MO, and is both of the PA and the MO can be used.

With the application to both PA and MO, it is possible to suppressparasitic oscillation more effectively, and to obtain an optical fiberlaser with a higher output.

EXAMPLES Example 1

An optical fiber laser was constructed as shown in FIG. 1A.

First, as a photonic bandgap fiber, an optical fiber was fabricated thathad the cross-sectional configuration as shown in FIG. 2A and thecharacteristics shown in FIGS. 2B, 2C. The core portion was doped withYb ions. The core portion was fabricated so that the wavelength regionin which a parasitic oscillation is most likely to occur, that is, thevicinity of 1030 nm to 1050 nm, which is a maximum gain wavelengthregion of the optical fiber doped with Yb ions, was excluded from thebandgap region, and that 1064 nm, which is a signal wavelength, was inthe bandgap region. To be more specific, pure silica glass, which wasfabricated into a core portion with a relative index difference Δc of 0%from pure silica glass and with a diameter d of 7.0 μm, was doped withYb ions. The core portion was coated with a first cladding made of puresilica glass with a diameter of 125 μm. Around a core portion of thefirst cladding, there were fabricated a plurality of high refractiveindex portions made of pure silica glass doped with germanium. The highrefractive index portions were arranged in triangular lattices in amanner spaced 7.0 μm away from each other, to thereby form a periodicstructure portion made of seven layers of high refractive indexportions. Each of the high refractive index portions had a maximumrelative index difference Ah of 2.8% from pure silica glass, and adiameter dh of 3.5 μm. The fabricated optical fiber had a coreabsorption amount of 1200 dB/m at a wavelength of 976 nm.

As for an optical coupler, one having a rare-earth-doped double-cladfiber with a core diameter of 7 μm and a cladding diameter of 125 μm asan exit port was used so as to make a connection loss with a Yb-dopeddouble-clad fiber small. As an excitation port, six multi-mode fiberswith a core diameter of 105 μm and an NA of 0.15 were used. As a signalport, a single-mode fiber with a core diameter of 7 μm and an NA of 0.14was used. As excitation light sources, six semiconductor lasers with anoscillation wavelength of 915 nm and a maximum output of 5 W were used.

In the optical fiber laser, the excitation light sources were drivenwithout a signal from the master oscillator to emit excitation lights,to thereby put the rare-earth-doped double-clad fiber in a state ofbeing excited with an excitation power of 30 W. After that, the time wasmeasured from the injection of the excitation lights to the occurrenceof a parasitic oscillation. The result is shown in FIG. 8.

As shown in FIG. 8, in the optical fiber laser of Example 1 using thephotonic bandgap fiber shown in FIGS. 2A to 2C, a parasitic oscillationdid not occur even with an excitation of approximately 30 μs.Furthermore, with the continuation of excitation for a long time, aparasitic oscillation was observed at approximately 70 μs.

Example 2

An optical fiber laser was fabricated similarly to Example 1, theexception being that the photonic bandgap fiber (hereinafter, sometimesreferred to as PBGF) shown in FIGS. 3A to 3C was used instead of thePBGF used in Example 1. In the PBGF of the present example, the firstlayer (the innermost first layer) of the periodic structure of the PBGFof Example 1 was removed, and pure silica was used instead. At thecenter, pure silica glass was doped with ytterbium oxide to befunctioned as an amplifying medium. Furthermore, the center was dopedwith aluminum oxide to form a core portion with a relative indexdifference of 0.3% from pure silica and with a diameter of 6 μm. Thehigh refractive index portions had a relative index difference of 2.6%from pure silica glass. Each of the high refractive index portions had adiameter of 4.8 μm, and the distance therebetween was 6 μm. The firstcladding was configured to have a diameter of 125 μm. A layer offluorine-based ultraviolet-curing resin with a relative index differenceof −0.5% from pure silica glass was arranged therearound. The PBGF usedin the present example was fabricated with a twist of one turn in 5 mm.

The PBGF used in the present example had a double clad construction inwhich light is guided in multimode with the first cladding made of puresilica as a second core, and a fluorine-based ultraviolet-curing resinlayer as a second cladding. The core absorption amount was 1200 dB/m ata wavelength of 976 nm.

In the optical fiber laser, the time from the injection of excitationlights to the occurrence of a parasitic oscillation was measured,similarly to Example 1. The result is shown in FIG. 9.

As shown in FIG. 9, in the optical fiber laser of Example 2 using thePBGF shown in FIGS. 3A to 3C, a parasitic oscillation did not occur evenwith an excitation of approximately 30 μs. Furthermore, with thecontinuation of excitation for a long time, a parasitic oscillation wasobserved at approximately 70 μs.

Comparative Example

Instead of the PBGF used in Example 1, a rare-earth-doped optical fiberwith a conventional double clad construction was used. The optical fiberhad a core doped with Yb ions, a core diameter of 6 μm, a first claddingdiameter of 125 μm, and a core absorption amount of 1200 dB/m at awavelength of 976 nm. In the optical fiber laser, the time from theinjection of excitation lights to the occurrence of parasiticoscillation was measured, similarly to in Example 1. The result is shownin FIG. 10.

As shown in FIG. 10, in the optical fiber laser using the conventionaloptical fiber, a parasitic oscillation was observed at approximately 30μs.

As described above, in the optical fiber laser of the present invention,it was confirmed that the use of the above PBGF 10 makes the time to aparasitic oscillation twice as long or longer than when compared withthe conventional optical fiber laser shown in Comparative Example. As aresult, the gain of the rare-earth-doped optical fiber immediatelybefore amplifying pulses is larger than that of the conventional fiberlaser. Therefore, it is possible to output pulses with higher energythan the conventional fiber laser.

Next, by use of the optical fiber laser of Example 1 fabricated as aboveand the optical fiber laser of Comparative example, pulses with a pulsewidth of 50 ns and a peak power of 60 W (a pulse energy of 0.003 mJ)were generated from the respective MOs, which were input to therespective optical fiber lasers (PAs). Then, maximum pulse energy thatwas output from each PA was measured. In the optical fiber laser ofComparative Example, a parasitic oscillation occurs at a pulse intervalof approximately 30 μs. Therefore, the interval of the pulses that enterfrom the MO were regarded as 30 μs. The result is shown in Table 1.

TABLE 1 Example 1 Comparative Example Pulse width 50 ns 50 ns Peak power(kW) 8 3 Pulse energy (mJ) 0.4 0.15

As shown in Table 1, in the optical fiber laser of Comparative Example,the pulses that were output from the PA had a pulse width of 50 ns and apeak power of 3 kW (a pulse energy of 0.15 mJ). In the optical fiberlaser of Example 1, the pulses had a pulse width of 50 ns and a peakpower of 8 kW (a pulse energy of 0.4 mJ). Therefore, according to theoptical fiber laser of the present invention, by use of theaforementioned PBGF, it is possible to obtain a power output with highenergy that conventional optical fiber lasers have not been capable ofachieving.

Comparative Example 2

Next, in fabricating the optical fiber laser of Example 2, a PBGFfabricated without applying twists was used to fabricate an opticalfiber laser similarly to in Example 2. This was used as an optical fiberlaser of Comparative Example 2. Similarly to Example 2, the time fromthe injection of the excitation lights to the occurrence of parasiticoscillation was measured. As a result, a parasitic oscillation wasobserved at approximately 40 μs.

This was because light in the vicinity of 1040 nm guided through theperiodic structure was not eliminated, and hence, the component of thelight slightly distributed in the core was amplified while being guidedthrough the PBGF of Comparative Example 2. In addition, this was becausethe gain obtained while the light was guided was larger than the loss ofthe light from the exit from the PBGF to the re-entry into the PBGFafter reflection off the isolator or the like.

A parasitic oscillation occurs at the time when the gain obtained in thePBGF becomes larger than the return losses on the entrance side and theexit side of the PBGF. That is, if the return losses on the entranceside and the exit side of the PBGF are larger than the loss obtained inthe PBGF, it is possible to suppress a parasitic oscillation. The returnlosses on the entrance side and the exit side of the PBGF are differentdepending on the circuit configuration of the optical fiber amplifier.In some cases, the return loss may be substantially zero. On the otherhand, when light enters another optical fiber from the PBGF, light withwavelengths outside the photonic bandgap region receives a great lossbecause most of its electric field is distributed in the periodicstructure.

Light guided through the PBGF receives a loss twice: when it exits thePBGF; and when it enters the PBGF. Therefore, if, when light exits fromor enters the PBGF, the loss of the light with wavelengths outside thephotonic bandgap region is more than half the gain when it is guidedthrough the PBGF, it is possible to suppress a parasitic oscillation.When the connection losses on the entrance side and the exit side weremeasured in the PBGF of Comparative Example 2, both were approximately15 dB. Therefore, it can be conceived that, if the connection losses onthe entrance side and the exit side of the PBGF are made to be greaterthan 15 dB, it is possible to suppress parasitic oscillation.

Example 3

The PBGF of Comparative Example 2 was modified to have an increasednumber only of layers of the periodic structure so as to allow morecomponents to be guided through the periodic structure, to therebyfabricate a PBGF capable of securing a connection loss of approximately25 dB on the entrance side and the exit side. The PBGF had a coreportion with a diameter of 6 μm, a first cladding with a diameter of 125μm, and a core absorption amount of 1200 dB/m at a wavelength of 976 nm.The PBGF was used to construct an optical fiber laser similarly to inExample 1. This was used as the optical fiber laser of Example 3. In theoptical fiber laser, the time from the injection of the excitationlights to the occurrence of a parasitic oscillation was measuredsimilarly to in Example 1.

As a result, a parasitic oscillation was observed at approximately 60μs.

Example 4

First, the PBGF shown in FIG. 6A was fabricated. The core portion issimilar to that of Example 1. Each of the high refractive index portionshad a diameter of 3.7 μm, and a relative index difference of 2.8% frompure silica glass. The distance therebetween was 7.3 μm. Next, as shownin FIG. 11, the PBGFs were taken out 1 m. Each of the PBGFs was woundone turn so as to make the diameter 100 mm, 80 mm, or 60 mmrespectively. After that, a white light source was irradiated onto anend face of each PBGF to excite the core portion. Then, transmissionspectra were measured. The results are shown in FIG. 12.

As shown in FIG. 12, light with wavelengths of approximately 1100 nm orlower was shut out, and light with wavelengths of approximately 1100 nmor higher was transmitted. Therefore, it has been confirmed that thePBGF used in the present example is capable of eliminating ASE lightwith wavelengths of 1000 to 1100 nm that is guided through the coreportion due to the filtering effect by the photonic bandgap, and is alsocapable of stably oscillating light with wavelengths of approximately1100 nm or higher.

Furthermore, the smaller the diameter around which the optical fiber waswound, the further on the long wavelength side the graph obtainedshifted. Therefore, with a change in the diameter when the optical fiberis wound, it was possible to easily control a gain profile of an opticalfiber for doping and amplifying rare-earth ions. Even if the diameteraround which the optical fiber was wound was changed, the filtering ofthe spontaneously emitted light at around 850 nm produced by theexcitation of yttrium was not influenced. That is, it has becomepossible to control a gain profile of an optical fiber for doping andamplifying rare-earth ions while the elimination effect on ASE light ismaintained.

Next, the optical fiber of the present example was used to build anoptical fiber laser similarly to Example 1. This was used as an opticalfiber laser of Example 4. The optical fiber used had a length of 19 m,with the diameter around which the optical fiber was modified waschanged from 60 mm to 100 mm. The optical fiber laser of Example 4 wasused to measure ASE spectra. The results are shown in FIGS. 13A, 13B.FIG. 13A shows a result when six semiconductor lasers with anoscillation wavelength of 915 nm and a maximum output of 0.45 W wereused as excitation light sources. FIG. 13B shows a result when sixsemiconductor lasers with an oscillation wavelength of 915 nm and amaximum output of 2.2 W were used as excitation light sources.

As shown in FIGS. 13A, 13B, it was possible to suppress ASE withwavelengths around 1030 nm. Furthermore, the smaller the diameter aroundwhich the optical fiber was wound was, the more effectively it waspossible to suppress ASE, and also the further on the long wavelengthside the peak of ASE shifted. From FIGS. 13A, 13B, it has been foundthat, even if the output of the excitation light source is changed, itis possible to control the gain profile of the optical fiber by changingthe size of the diameter around which the optical fiber was wound.

INDUSTRIAL APPLICABILITY

The present invention is applicable to an optical fiber laser capable ofstably emitting pulses with high energy.

1. An optical fiber laser comprising: a master oscillator which is alaser oscillator producing a seed beam; and a power amplifier which isan optical amplifier connected to a subsequent stage of the masteroscillator and amplifying the seed beam emitted from the masteroscillator and outputting a laser beam, wherein the power amplifiercomprises: a plurality of excitation light sources; excitation portseach of which is connected to the excitation light sources and which anexcitation light emitted from each of the excitation light sourceenters; a signal port which a laser beam emitted from the masteroscillator enters; an optical coupler with an exit port that outputs theexcitation lights from the excitation ports together with the laser beamfrom the signal port; and an optical fiber connected to the exit port,wherein the optical fiber is a photonic bandgap fiber, and wherein theoptical fiber has a loss wavelength characteristic in that a photonicbandgap region is narrower than a gain wavelength band in a graph withan axis of abscissa representing a wavelength and an axis of ordinaterepresenting a loss amount, wherein the optical fiber comprises: aregion; a core portion made of a solid material doped with a rare-earthelement, provided in the region, and having a higher refractive indexthan the region; a first cladding provided around the region; and aperiodic structure portion in which a multitude of high refractive indexportions with a refractive index higher than that of the first claddingare arranged in a periodic structure, the periodic structure portionbeing provided in a vicinity of the region in the first cladding.
 2. Theoptical fiber laser according to claim 1, wherein a maximum relativeindex difference of the high refractive index portion is 2% to 3% withrespect to the first cladding.
 3. The optical fiber laser according toclaim 1, wherein the core portion is higher than the periodic structureportion in electric field distribution of light with wavelengths in thephotonic bandgap region; and the periodic structure portion is higherthan the core portion in electric field distribution of light withwavelengths outside the photonic bandgap region.
 4. The optical fiberlaser according to claim 1, wherein at least germanium is included inthe high refractive index portion.
 5. The optical fiber laser accordingto claim 1, wherein the core portion is doped with Al or Ge.
 6. Theoptical fiber laser according to claim 1, wherein the core portion has arelative index difference of approximately 0.1 to 0.5% from the region.7. The optical fiber laser according to claim 1, wherein a maximum gainwavelength region of the optical fiber doped is excluded from a bandgapregion.
 8. The optical fiber laser according to claim 1, wherein anoptical loss is inflicted on light which is guided through the coreportion in the wavelength region outside the photonic bandgap.